Trouble diagnosing an AdSense Viewability / Active View issue

I’m having a viewability issue with a vertical Adsense placement (300×600 and under).

It’s a pretty standard above-the-fold, right column placement. The top of the placement starts around 200 px from the top of the browser window, so it’s a very visible placement. Easily the most visible on the site for desktop users.

The placement is hidden in mobile (by hiding the parent per current AdSense regulations) and doesn’t serve to mobile devices. (This is tested and confirmed, btw… so that’s not the issue).

The placement is currently scoring a meager 30% Active View in AdSense reports, which means it’s serving and not being seen — a lot.

Where else should I be looking to diagnose this problem? I’m pretty sure the viewability issue is negatively impacting RPM.

The site has excellent traction, with over 6 ppv and an average pageview time of nearly 1 minute. The bounce rate is a very healthy 33%. The CWV on the page are all healthy (Rather, as healthy as can be expected with a site running AdSense).

Anyone familiar with viewability / active view issues? Any thoughts on where else the issue could lie?

Thanks.